Compound Nucleus Process Research Articles

A study of the12C(6Li,p)17O reaction at E=28 MeV

States in 17O have been populated using the 12C(6Li,p)17O reaction at 28 MeV. Some levels are strongly excited and it is suggested that they may contain substantial 5p-4h configurations. Angular distributions show characteristics of both compound nucleus and direct processes and although the overall magnitude of the cross section for most levels is well reproduced by Hauser-Feshbach calculations the forward angle data for the ground state have been fitted assuming a direct transfer of five nucleons as a 5He cluster.

Study of the 6Li + 16O light heavy-ion system around the Coulomb barrier

Total cross sections of the light heavy-ion reactions 16 O( 6 Li, p i) 21 Ne, i = 0–10 , have been measured for beam energies E Li = 4.5–8.0 MeV in steps of 100 keV. Additional excitation functions of the inclusive reactions 16 O( 6 Li, xγ), x = p, n and α , were taken for E Li = 4.1–12.5 MeV. The 6Li + 16O reaction can be understood as a predominant compound-nucleus process as extensive Hauser-Feshbach calculations show. A comprehensive statistical analysis yields a coherence width Γ = 130 ± 20 keV for E x ( 22 Na) ≈ 19 MeV . Statistically significant deviations from pure fluctuation phenomena are found in most excitation functions at E x ( 22 Na) = 17.9, 18.4, 19.2 and 20.2 MeV . Interpreting these structures with widths 400 ≤ Γ tot ≤ 800 keV as intermediate-width resonances in terms of a rotational band, tentative spin assignments can be given.

Fast Neutron Inelastic Scattering Cross Sections of238U for States between 680 and 1530 keV

Neutron inelastic scattering cross sections for /sup 238/U levels between 680- and 1530-keV excitation energy have been measured in the incident neutron energy range from 0.9 to 2.2 MeV. The (n,n') time-of-flight (TOF) technique was used to obtain direct differential inelastic cross sections. Neutrons were generated using the /sup 7/Li(p,n)/sup 7/Be reaction. Experimental parameters were optimized to achieve an energy resolution of <15 keV. Level cross sections were deduced from the measured 125-deg differential scattering cross sections. The validity of this procedure was confirmed by measuring the angular distributions for nine levels at En=1.5 and 2.0 MeV. Background due to fission induced by fast neutrons was subtracted. The TOF spectra were unfolded using the method of the response function. The data were corrected for multiple scattering and neutron attenuation in disk scattering geometry using an analytic method. Theoretical calculations of the cross sections were carried out using reaction models appropriate to the description of compound nucleus and direct interaction processes. The data are compared to (n,n'..gamma..) results and the ENDF/B-V evaluation.

Imprints of compound-nucleus and direct-interaction processes in energy-averaged cross sections and durations of nuclear reactions

New analytical expressions for the energy-averaged S-matrix and nuclear-reaction cross sections are obtained on the basis of the unitary S-matrix parametrization. Some approximations are analyzed when compound-nucleus cross sections are described with and without the Hauser-Feshbach formula with the partial-width-fluctuation correction. The connection of the S-matrix parameters in the Simonius representation with the dynamics of compound-nucleus processes is clarified. An explicit expression for the averaged duration of the nuclear reaction is obtained for the range of closely situated resonances. Possible methods of obtaining information on some compound-nucleus and direct-interaction characteristics from the averaged cross sections and durations are examined.

Fast neutron radiative capture in silicon

Using the28Si(n, γ)29Si reaction, transitions to the ground state and first excited state in29Si have been studied in the neutron energy range 3–14 MeV with improved neutron energy resolution (of about 100 keV). The 90° cross sections show considerable structure in the entire neutron energy range. Comparison with theoretical calculations shows that compound-nucleus and direct-semidirect processes account for the non-resonant part (smoothly varying part) of the cross section. A microscopic model is, however, required to describe the resonance structure. Continuum shell-model calculations have proven to be a very promising means towards a better understanding of the capture process in, and below, the giant resonance region in light nuclei. The angular distributions of gamma rays in the neutron energy range 8–14 MeV indicate that the capture reaction is mainly of direct character and that the effect of interference between the electric dipole and isoscalar quadrupole resonance is weak.

Program Summary

The angular distributions of gamma rays following inelastic neutron scattering near threshold to the first excited states of the target nuclei were investigated experimentally and theoretically.Measurements used NaI(Tl) crystal detectors and natural targets of iron and cerium. Data were obtained for the angular distribution of gamma rays following inelastic neutron scattering by 56Fe at the neutron center-of-mass energies E = 0.87, 0.90, 0.93, 0.95 and 0.99 MeV and by 140Ce at E = 1.64, 1.67, 1.71, 1.75 and 1.87 MeV.The predictions for the gamma-ray angular distributions of the two following theories are determined: (i) the Satchler-Scheldon theory and (ii) the theory of Moldauer, specialized to the pure compound nucleus process and employing the “uncorrelated amplitude” assumption. Near threshold the predictions of the two theories are nearly the same.Theory and experiment are in good agreement for cerium at all energies at which angular distributions were obtained and for iron at E = 0.99 MeV but not at lower energies.Two general conclusions are drawn about measurements of the angular distribution of gamma rays following inelastic neutron scattering in medium and heavy nuclei at energies near threshold; (i) such measurements are not useful in deciding between the theories of Satchler-Sheldon and Moldauer, but (ii) they provide data for a sensitive test of the Hauser-Feshbach statistical assumptions.

A study of the30Si(d, p)31Si reaction at 10 and 17 MeV

Angular distributions have been measured for the 30Si(d, p) 31Si reactions and for 30Si(d, d) 30Si elastic scattering at 10 and 17 MeV. Fourteen states not previously observed in 31Si are reported. The l assignment for the state at 6.111 MeV (l=(1, 2)) is different from that obtained previously. Investigations of the reaction mechanism indicate that contributions from single- and multistep direct reactions and compound-nucleus processes are necessary to account for the measured cross sections. It is also found that neither the magnitude nor the distribution of the measured spectroscopic strength can be reproduced by shell-model calculations using a full (s, d) basis.

Fusion and direct reactions for strongly and weakly bound projectiles

The interaction of 6Li, 9Be and 12C projectiles with a 28Si target was investigated by measuring the angular distributions of the elastically scattered projectiles and of the emitted protons, deuterons and α-particles. The experiment was performed in order to deduce direct and compound nucleus process contributions to the total reaction cross section and to study the influence of the projectile structure on the relative importance of these two mechanisms. Optical model parameters and therefore the total reaction cross section are strongly influenced by the binding energy of the projectile. The parameters of the Glas-Mosel model describing the fusion reaction vary smoothly with the atomic number. In the system 9Be + 28Si around 50% of all reactions are direct processes even at energies near the Coulomb barrier, whereas in the other systems the direct part amounts to 15 % ( 12C) and 30 % ( 6Li) only.

Statistical analysis of the (d, p) and (d, d) reactions on 40Ca

Excitation functions of the 40Ca(d, p) 41Ca and 40Ca(d, d) 40Ca reactions have been measured at 45°, 90°, 135° and 170° from E d = 4.50 to 5.43 MeV in 10 keV steps. Angular distributions of these reactions have been taken at E d = 4.70, 5.00 and 5.30 MeV from 25° to 170° in 5° steps. Transitions were observed to the excited states for the range 0.0 ≦ E x ≦ 3.74 MeV in 41Ca. Rapid fluctuations in the excitation functions and strong variations of the angular distributions with the incident energy were observed, suggesting that the contribution from compound nucleus processes is very large. Various quantities extracted from the experimental data were compared to the predictions of the statistical theories combined with the DWBA theory for the calculation of the direct reaction amplitudes. The results of the present analysis are consistent with the predictions of the standard statistical theories based on the neglect of the channel-channel correlation.

Time delays in reactions with intermediate and compound nuclear structure

We present a simple model, analytically solvable, of the time evolution of the scattering of a wave packet, which exhibits and clarifies the compound nucleus process, and can be easily extended to the study of the time delays involved when intermediate structure is present.

Implications for Radiative-Strength Functions from Neutron Capture inCl35

A comparison of neutron-capture $\ensuremath{\gamma}$-ray spectra in neighboring $s$- and $p$-wave-capture states in a $^{35}\mathrm{Cl}$ target shows two sharp departures from what is normally expected from a compound-nucleus process. The spectra are highly correlated, with $r=+{0.84}_{\ensuremath{-}0.10}^{+0.16}$, and $E1$ and $M1$ strengths are comparable. The $E1$ strength can be explained by the $E1$ giant resonance; it is consistent with the Axel prediction for a ${A}^{\frac{8}{3}}$ dependence. $M1$ strengths agree with single-particle estimates, in sharp contrast to heavy nuclides.

A study of the 14N(d, p)15N reaction at low bombarding energies

Absolute differential cross sections of the proton groups p0, p1 + p2, p3, p4 and p5 from the 14N(d, p)15N reaction have been determined at bombarding energies between 0.309 and 0.638 MeV. The shape of the measured angular distributions are generally well reproduced by the incoherent sum of the contributions of direct and compound nucleus processes calculated with the distorted wave Born approximation and the Hauser-Feshbach formula respectively. However, a reliable value for the spectroscopic factor can be extracted only for the p5 transition whose mechanism even at this energy is almost entirely direct.

Angular Distributions of Neutrons from Pre-Compound Decay

Angular distributions of neutrons from pre-compound decay are resolved from those of compound-nucleus process for the 93 Nb ( n , n ') reaction induced by 14 MeV neutrons. Comparisons are made with the calculated results and good agreements are obtained.

Compound nucleus reactions of B with Ta and Ag

Excitation functions and mean recoil ranges are reported for the formation of 188Pt, 186Ir, 185Os in 10, 11B reactions with 181Ta and for 113Sn in 10, 11B reactions with 107–109Ag. The stacked foil technique was used. The results are indicative of a compound nucleus process. Mean recoil ranges are compared to theoretical calculations with corrections for multiple scattering. The agreement is satisfactory. Evaporation calculations were performed with a modified statistical model code GROGI 2. A good fit is obtained with parameters derived from studies of nuclei at low excitation energies; no enhancement of the rate of γ-emission is needed.

Study of theB10(O16,α)Na22reaction

States in $^{22}\mathrm{Na}$ were populated using the $^{10}\mathrm{B}(^{16}\mathrm{O},\ensuremath{\alpha})$ reaction. Excitation functions were measured in the energy range ${E}_{\mathrm{c}.\mathrm{m}.}=15.4\ensuremath{-}17.7$ MeV for 47 excited states in $^{22}\mathrm{Na}$ from 0 to 14 MeV excitation. Angular distributions were measured at a bombarding energy of 46 MeV for a number of the excited states and were observed to be symmetrical around 90\ifmmode^\circ\else\textdegree\fi{} (c.m.). Fluctuation analyses of the excitation functions were made, and the results indicated a statistical compound-nucleus process for the reaction mechanism. Hauser-Feshbach calculations gave good agreement with the data and provided strong evidence for proposed high-spin members of the ground-state rotational band, and the $K={0}^{+}$, $T=0$ band in $^{22}\mathrm{Na}$.[NUCLEAR REACTIONS $^{10}\mathrm{B}(^{16}\mathrm{O},\ensuremath{\alpha})$, ${E}_{160}=40 \mathrm{to} 46$ MeV, measured $\ensuremath{\sigma}(E)$ for ${\ensuremath{\theta}}_{\mathrm{lab}}={7}^{\ensuremath{\circ}}$; ${E}_{^{16}\mathrm{O}}=46$ MeV, measured $\ensuremath{\sigma}(\ensuremath{\theta})$; $^{22}\mathrm{Na}$ deduced levels. Fluctuation analysis. Hauser-Feshbach calculations and suggested $J$ values. Enriched targets.]

Neutron total and scattering cross sections of niobium in the continuum region

Neutron total and elastic and inelastic scattering cross sections of niobium are measured in the incident energy interval 0.25 to 0.5 MeV. Total neutron cross sections are determined to a few percent accuracy throughout the energy interval. Differential elastic and inelastic neutron scattering cross sections are measured at ≈ 20 scattering angles distributed between 20 and 160 degrees and at incident neutron energy intervals of 0.1 MeV from 1.8 to 4.0 MeV. The cross sections for the inelastic neutron excitation of 11 states withQ- values of → 2520 keV are quantitatively measured and the excitation of an additional 8 states qualitatively observed. Calculations based upon a selected optical potential give a good description of total neutron cross sections to 20.0 MeV and of elastic scattering cross sections extending from low energies where compound-nucleus processes are prominent to high energies where “shape” scattering predominates. Inelastic neutron scattering cross sections are calculated from the optical potential and statistical theory and comparison with the corresponding experimental values suggests the spins and parities of the low-energy excited states.

Isomerism in 69Se

Isomeric states of 69Se were produced via the heavy-ion compound-nucleus process and from the positon decay of 69Br, the former producing a high-spin state and the latter preferentially populating a low-spin state. Ge(Li) detectors as well as scintillation counting showed that the high-spin state decays with T 1 2 = 14.0 ± 1.0 min to levels in 69As, giving rise to γ-rays of 116, 166 and 282 keV. The low-spin state decays predominantly to a state giving rise to a 145 keV γ-ray with T 1 2 = 1.8 ± 0.2 min. The beta end-point energy of both isomers is 5.2 ± 0.2 MeV. It is deduced that the high-spin state, assigned J π = 9 2 +, is the ground state; the isomeric state had J π = 3 2 −. The angular momentum inversion as compared to neutron-excess Se isotopes is attributed to Coriolis coupling to a deformed core as described by Malik and Scholz.

C12(O16,α)Mg24Excitation Functions and Statistical-Model Calculations for High-Spin States inMg24

High-resolution excitation functions were measured in the energy range ${E}_{\mathrm{c}.\mathrm{m}.}=19\ensuremath{-}25$ MeV for the $^{12}\mathrm{C}(^{16}\mathrm{O},\ensuremath{\alpha})$ reaction to highly excited states in $^{24}\mathrm{Mg}$ (${E}_{\mathrm{exc}}\ensuremath{\approx}13.5\ensuremath{-}17.5$ MeV). The strong, rapid observed fluctuations suggest a predominately compound-nucleus reaction mechanism. The selective population of high-spin states in $^{24}\mathrm{Mg}$ by the present reaction in contrast to the lack of such selectivity in the $^{14}\mathrm{N}(^{14}\mathrm{N},\ensuremath{\alpha})$ reaction can readily be explained by Hauser-Feshbach statistical-model calculations. A fluctuation analysis indicates coherence widths of 90-150 keV, and cross correlations were found to be small and thus compatible with a purely statistical compound-nucleus process.

Reaction mechanisms and shell structure effects in 54Fe+ 6Li and 58Ni+d reactions at medium energies

Excitation functions and average recoil ranges for products formed in deuteron induced reactions on 58Ni and in 6Li ion induced reactions on 54Fe have been measured. The incident energies considered ranged from 4 to 40 MeV for the deuterons and from 15 to 60 MeV for the 6Li ions. The recoil range values indicate the predominance of equilibrium compound nucleus processes at energies up to several MeV beyond the peaks in the excitation functions. At still higher energies a statistical model analysis of the data indicates the increasing importance of pre-equilibrium particle emission. In the analysis, the consideration of shell effects according to Rosenzweig's model leads to greatly improved estimates of the experimental isobaric yield ratios. Evidence for direct reaction processes leading to the formation of 57Ni and 56Ni in the deuteron induced reactions and to 55Co in the lithium ion induced reactions has also been observed. The validity of the independence hypothesis is verified for excitation energies at which the recoil ranges indicate the predominance of equilibrium statistical processes in both reaction systems.

The Photonuclear Reactions Leading to 24Na and 28Mg

Abstract The photonuclear reaction yields of 24Na and 28Mg in magnesium, aluminum, silicon, phosphorus, and sulfur were obtained at energies between 20 MeV and 250 MeV by the induced radioactivity method. The 12C(γ,n)11C yield was used as a standard for the beam monitor. The excitation curves of the 27Al→24Na, 28Si→24Na, Si30→28Mg, 31P→24Na, 31P→28Mg, 32S→24Na, and 32S→28Mg reactions were obtained from the yields by the photon-difference method. Each of these excitation functions indicates a peak in the 50–70 MeV energy range, except in the case of the sulfur target, in which the peak is due to the contribution from the compound-nucleus process. The interaction of photons with nuclei at photon energies exceeding the peak energy region probably does not lead to the formation of the compound nucleus. In the energy range from 70 MeV up to 150 MeV, the cross sections are due to the contribution from the quasi-deuteron process. At energies above 150 MeV and up to 250 MeV, the quasi-deuteron and pion production processes compete. In the high-energy photonuclear reaction mechanisms, the quasi-deuteron and pion production processes must be considered at the initial cascade process, but the evaporation process may be considered after the cascade process.